Strategies

Heterotrophic animals such as predators that derive their energy from other organisms (prey or hosts) can be classified by their consumer-resource interactions with those organisms.[1] A perspective on the evolutionary options available to predators and parasites can be gained by considering four questions: the effect on the fitness of the prey or host; the number of prey or hosts they have per life stage; whether the prey or host is prevented from reproducing (by being killed, or by being castrated), reducing its evolutionary fitness to zero; and whether the effect depends on intensity. From this analysis, the major evolutionary strategies of predation and micropredation emerge, alongside parasitism and parasitoidism; social predators such as lion and wolf are distinguished from solitary predators like the cheetah.[2]

Grazing and micropredation

Grazing animals generally do not kill their prey, but like predators, they live by feeding on other organisms. While some herbivores like zooplankton live on unicellular phytoplankton and therefore inevitably kill what they eat, in a relationship sometimes called predation,[14] many others including cattle and sheep only eat a part of the plants that they graze.[15] Many species of plant are adapted to regrow after grazing damage. For example, the growing meristems of grasses are not at the tips as they are in most flowering plants, but at the base of the leaves.[16] Similarly, kelp is grazed in subtidal kelp forests, but continuously regrows from a meristem at the base of the blade where it joins the stipe.[17]Herbivore-plant interactions, as with predator-prey interactions, have driven plants to evolve defences such as thorns and chemicals to dissuade grazing.[4]

Parasitism

Parasites, like predators, live by feeding on another organism, but differ in that they often do not kill their hosts. The entomologist E. O. Wilson has characterised parasites as "predators that eat prey in units of less than one".[20][4][1][18]

Parasitoidism

Parasitoids are insects living in or on their host and feeding directly upon it, eventually leading to its death, making their strategy comparable with predation. They are, however, much like parasites in their close associations with their hosts. Unlike typical parasites, they always kill their hosts, but often not instantly. Parasitoid wasps are solitary insects that live a free life as adults, laying eggs on or in other insects such as lepidopteran caterpillars. The wasp larvae feed on the growing host, eventually killing it. Parasitoids make up as much as 10% of all insect species.[21][22]

Social predation

In social predation, a group of predators cooperates to kill creatures larger than those they could overpower singly. Social predators such as lions, hyenas, and wolves collaborate to catch and kill large herbivores. By hunting socially chimpanzees can catch colobus monkeys that would readily escape an individual hunter, while cooperating Harris hawks can trap rabbits.[23][24][25]